Investigators are focusing on the kitchen as they seek the starting point of Friday's fire and the cause of the roof collapse that killed four Houston firefighters and injured many more.

Ed Arthur, deputy chief of Houston Fire Department's Arson Division, said at a news conference Tuesday that it is too early to determine a specific cause, but said the multi-agency team so far has focused its attention on the area above the kitchen in the restaurant attached to the Southwest Inn in the 5800 block of the Southwest Freeway. Witnesses have said the fire started in the restaurant and spread to the motel.

Arthur confirmed reports that the roof of the adjoined buildings had multiple layers, possibly the result of reconstruction and repair work.

"That may have been a contributing factor," he said.

He declined to answer many specific questions about the building and the investigation, such as whether there were sprinklers or grease build-up could have contributed, saying they will spend at least another week collecting data from the scene. Arthur said it would take months to organize and interpret that information before each agency releases its report.

The agencies began their work at the scene Saturday. ATF used lasers to map what is left of the building Sunday, and on Monday crews began removing debris so investigators could look more closely at some walls.

Arthur said more than 100 witnesses have been interviewed to date, including more than 60 firefighters who responded that day. He encouraged anyone with information about the fire, including photos or videos, to contact the arson division at (713) 247-8900.

Noting that it is the third time he has investigated firefighter deaths, Arthur said it has been difficult work because many working the scene knew the four who died.

"It's an added commitment for them to get to the bottom of this fire," he said.